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Macrophage activation syndrome 

Macrophage activation syndrome
Macrophage activation syndrome
Oxford Textbook of Rheumatology (4 ed.)

Alexei A. Grom

and Athimalaipet V. Ramanan

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Macrophage activation syndrome (MAS) is a life-threatening condition caused by excessive activation and proliferation of T lymphocytes and haemophagocytic macrophages. Although MAS has been reported in association with almost any rheumatic disease, it is by far most common in systemic juvenile idiopathic arthritis. Flares of the underlying disease or infection are most common triggers of MAS. The pathognomonic feature of MAS is typically found in bone marrow: numerous, well-differentiated macrophagic histiocytes phagocytosing normal haematopoietic elements. The expansion of these histiocytes leads to a massive systemic inflammatory reaction associated with three cardinal clinical features: severe cytopenias, liver dysfunction, and coagulopathy consistent with disseminated intravascular coagulation. Clinically, MAS is strikingly similar to the autosomal recessive disorders collectively known as familial haemophagocytic lymphohistiocytosis (FHLH). FHLH has been associated with various genetic defects affecting the cytolytic pathway. Cytolytic function is profoundly depressed in MAS patients as well, and this abnormality is caused by both genetic and acquired factors. Studies in animals suggest that uncontrolled expansion of activated CD8+ T lymphocytes secreting cytokines that activate macrophages is central to the pathophysiology of haemophagocytic syndromes. Consistent with this view, the combination of steroids and ciclosporin, an immunosuppressant that preferentially inhibits T lymphocytes, is an effective treatment for the majority of MAS patients. Patients in whom MAS remains active despite this treatment present a serious challenge and require more aggressive immunosuppression. However, in MAS triggered by infection, the optimal level of immunosuppression is difficult to determine. As a result, reported mortality rates reach 20%.

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